You know, I was browsing through my manga collection the other day when this exact question popped into my head. 'One Punch Man' is such a powerhouse in the anime and manga world, but its light novel presence is surprisingly understated. While the series primarily shines through its manga and anime adaptations, there isn't a dedicated light novel series like you'd see with 'Sword Art Online' or 'Overlord'. However, there are a few novelized spin-offs and side stories that dive deeper into the Hero Association's bureaucracy or random S-class heroes' antics. They're fun reads if you're craving more of the OPM universe, but they don't follow Saitama's main journey.
What's interesting is how these side stories flesh out the world. One I particularly enjoyed was 'One Punch Man: Hero Nobody Knows', which explores lesser-known heroes and their struggles—almost like a parody of typical shonen backstories. It's a neat complement to the main series' satire, though it lacks the punch (pun intended) of Yusuke Murata's iconic artwork. If you're a completionist, they're worth checking out, but don't expect them to replace the manga's sheer brilliance.